The present invention relates to methods and devices for continuous on-line and discrete off-line monitoring of organic compound contaminants in fluid streams. The present invention is particularly concerned with monitoring concentrations of such contaminants in terms of recognized categories based on different analytical techniques: total organic carbon (TOC), non-methane organic carbon (NMOC), and volatile organic compounds (VOCs). The fluid streams-in which such organic compound contaminants can be found, and in which their presence must be detected and their concentrations measured, are, e.g., those fluid streams emitted from industrial and commercial stacks, those discharged as effluents from air toxic control devices and as waste water from plants, and those present in drinking water sources.
Organic compound contaminants, especially volatile and volatilizable organic compounds (VOCs) in the environment may be hazardous to public health even at very low concentrations, since many of the VOCs are toxic, mutagenic, and/or carcinogenic, such as aromatic and halogenated compounds. Organic compound contaminants which are xe2x80x9cvolatilexe2x80x9d, as that term is used in the present invention, are those which have a relatively high vapor pressure and can be found in vapor form at relatively low temperatures. However, there is also included within the definition of xe2x80x9cvolatile organic compoundsxe2x80x9d (VOCs), as that term is used in the present invention, organic compounds which are xe2x80x9cvolatilizablexe2x80x9d i.e., capable of being made volatile. Such volatilizable organic compounds are particularly those which may vaporize under the conditions of concentration and detection to be found during the methods of the present invention. The measurement of VOCs in air has become a very important goal. Conventional VOCs monitoring methods involve collecting a sample at the site and transporting it to the laboratory for analysis. While these methods are quite accurate, they cannot be utilized for continuous on-line analysis to provide information on a real-time basis as required for effective pollution control and for meeting regulatory requirements. A discussion of such methods, and of the state of the art relating to them, can be found in U.S. Pat. No. 5,435,169, which is incorporated herein by reference in its entirety.
In accordance with the present invention, analytical apparatus and instrumentation is provided which can be used in various aspects of continuous on-line measurement of organic compound contaminants, and which can also be used off-line for isolated, i.e., discrete measurements which may be single instances or repetitive occurrences. The objective of the analytical apparatus and instrumentation is three-fold. The organic compound contaminants are to be concentrated to facilitate detection of low concentrations; interfering species such as CO2 and H2O are to be eliminated; and then the trapped organic compound contaminants are to be injected into the detector. A variety of detector systems may be used with this analytical approach, e.g., non-methane organic carbon analysis (NMOC), total organic carbon analysis (TOC), mass spectrometry (MS), infrared spectroscopy (FTIR), or any other suitable detection system. Systems of this type can be used on-line to monitor emissions from industrial stacks, vents and similar sites from which emissions originate.
Total Organic and Non-Methane Organic Carbon Analysisxe2x80x94Total organic carbon is a measure of total carbon emissions in organic form, i.e., the total carbon content less that derived from the permanent gases such as CO2 and CO. Non-methane organic carbon (NMOC) is another category of organic compound contaminant measurement frequently used in addition to total organic carbon, and is a measure of the total organic carbon content of a sample, except that coming from methane. In non-methane organic carbon analysis, methane, CH4, is treated as a permanent gas, although it is not treated as a permanent gas in the other analyses.
In the mid-1970""s, EPA Standard Method 25 was developed for quantifying NMOC emissions from stationary sources. In that method, gas samples are collected and sent to a lab for analysis. In a conventional non-methane organic carbon analyzer, one milliliter of gas sample is introduced into a separation column through a gas sampling valve. The column is used to separate VOCs from permanent gases such as CO2, CH4 and CO. After the gases elute from the column, i.e. , a CO2 peak appears, the column is backflushed into the detector system and all of the organics are then measured together as one peak.
The detection system comprises an oxidation unit, a reduction unit and a flame ionization detector (FID). The reason for converting all of the organic compounds to CH4 is that different compounds have different response factors in PID, and in this manner a response directly proportional to the number of carbon atoms is obtained.
The use of column separation in conventional NMOC analysis poses significant problems especially when the sample contains large amounts of moisture and the concentration of CO2 is above 8% by volume of the sample. Another major problem is that the detection limits are not low enough, as a result of the fact that the injection volume must be limited in order to obtain good separation in the column. Another drawback of this method is that it cannot be used for continuous on-line monitoring. Other total organic carbon analysis methods are also used where, instead of reducing the CO2 to methane, the CO2 itself is measured using infrared or other suitable detection means.
Continuous On-Line FID, MS and FTIRxe2x80x94At present on-line analysis is done using a flame ionization detector (FID) for total hydrocarbon analysis. Similarly, the mass spectrometer (MS) and the Fourier Transform Infrared Spectrophotometer (FMIR) are used for on-line VOCs monitoring. In the case of both the FID and the MS, the sample is introduced directly into the detector. No sample concentration is used, and thus the detection limits are quite high. However, H2O, CO and CO2, which are always present in environmental emissions, interfere in the analysis. In the case of the FIR, the absorbance spectra is measured in a flow cell, or else a long path FTIR is used in which the IR beam is reflected across the emission source. Here also, the presence of H2O, CO and CO2 can also interfere with the analysis.
Monitoring VOCs in Waterxe2x80x94Most conventional VOCs monitoring is done by using the purge and trap method. Typically, the sample is collected in the field and then transported to the laboratory. On-line purge and trap systems have also been developed for semi-continuous monitoring.
In accordance with the present invention, devices and methods are disclosed for the continuous, substantially real-time, monitoring of very low level concentrations, i.e., levels below parts per million by volume (ppm), and even levels below parts per billion by volume (ppb), of organic compound contaminants for environmental and chemical process, monitoring and control purposes. The device has general utility in effectively continuous on-line monitoring using different detection devices such as those for non-methane organic carbon (NMOC) and total organic carbon (TOC); specific devices, e.g., the mass spectrometer (MS) and the Fournier Transform Infrared Spectrophotometer (FTIR); total hydrocarbon analysis using a Flame Ionization Detector (FID), Gas Chromatograph (GC) associated detectors; and other detector means, e.g., the Electrolytic Conductivity Detector (ELCD), Electron Capture Detector (ECD), Thermionic Ionization Detector (TID), Nitrogen Phosphorous Detector (NPD), Flame Photometric Detector (FPD), and Thermal Conductivity Detector (TCD), and so forth.
The key operating feature of the method of the present invention is the use of an adsorbent trap means for the organic compound contaminants, which may be based, e.g., on the differential adsorbent capacity of compositions such as activated carbon, on the differential temperature effects of cryogenic processes, etc., to capture and retain the organic compound contaminants while the interfering and background species, e.g., H2O, CO2, CO, etc., are permitted to pass through. The organic compound contaminants are then released quickly from the adsorbent trap means, preferably through a rapid desorption achieved by the heating of the adsorbent trap means. Following desorption, the sample is injected into one of the detector means identified above, e.g., by the use of an inert carrier gas.
The devices and methods of the present invention are useful for the detection and measurement of organic compound contaminants in fluid sample streams. The fluid stream may be either gaseous, e.g., air, or liquid, e.g., water. Where the sample stream is gaseous, it is preferred that the detector means be other than gas chromatography (GC). Gas chromatography is, properly, a means for separating the components of a sample, and a detector means associated therewith may be used to xe2x80x9creadxe2x80x9d the results, i.e., to physically identify the separated components. It has been found that by using the adsorbent trap means of the present invention and rapid desorption of the organic compound contaminants therefrom, that it is not necessary to employ gas chromatography separation, as required in the methods of the prior art. A gas chromatographic separation step may be included, however, and is preferred in the methods of the present invention where the fluid sample stream is water or a liquid effluent containing or suspected of containing organic compound contaminants.
In U.S. Pat. No. 5,435,169, the disclosure of which is incorporated herein by reference in its entirety, a method and corresponding apparatus for the continuous, on-line GC analysis of VOCs is disclosed, which involves the collection of VOCs in a concentrator element, and the separation of the VOCs from the permanent gases, followed by the introduction of the sample to a gas chromatography (GC) column. The concentrator element is an adsorbent trap means containing at least one adsorbent that is able to separate the organics by adsorption, while venting the permanent gases. The concentrator is purged by desorption and the desorbed gases are then injected into the GC and associated detector. This process is performed rapidly and frequently in order to achieve effectively continuous on-line monitoring. This process and the corresponding apparatus are outlined in U.S. Pat. No. 5,435,169 with respect to VOCs and the injection of the concentrated material into a GC. However, the need for greater analytic simplicity, together with extended functioning of the adsorbent trap means for separation of a more specific array of interfering materials, has led to its implementation with additional detector means such as FID, MS, FTIR, ELCD, ECD, TID, NPD, FPD, and TCD, and additional applications such as the monitoring of NMOC and TOC concentrations. The resulting improved method and corresponding apparatus for continuous on-line use of the present invention will now be described.
Application in NMOC, FID, MS and FTIR Monitoringxe2x80x94The method of the present invention is quite different from the conventional on-line NMOC, TOC, FID, MS or FTIR analysis. The sample containing the organic compound contaminants is passed directly through an adsorbent trap, which selectively retains the volatile organics while allowing the other gases, e.g., CO, CO2, O2, N2, SO2, and NO2, as well as moisture, to pass through. Then the trap is rapidly heated by electrical, microwave or other heating sources to desorb the organic compound contaminants into the detection system. The adsorbent trap is designed to heat and cool very rapidly, so that injections can be made very frequently, e.g., every few seconds to every few minutes. Substantially continuous analysis is possible by repeating this cycle frequently. The trap serves three purposes: (1) trapping and concentrating the organic compound contaminants; (2) separating moisture and the permanent and other gases from the sample; and (3) injecting the organic compound contaminants into the detector instrument. Measurements at very low concentration levels are possible because the pollutants are concentrated within the trap, which can be used by itself or in conjunction with other injection devices, especially a conventional sample valve.
Monitoring Organic Compound Contaminants in Waterxe2x80x94The method and device of the present invention can also be used for the monitoring of organic compound contaminants in water. The aqueous sample containing the organic compound contaminants passes through a membrane module in which the organic compound contaminants selectively migrate across the membrane into an inert gas stream. The organic compound contaminants are trapped and concentrated by an adsorbent trap means. The retained organic compound contaminants are desorbed from the adsorbent trap means by an electrically generated heat pulse which also serves as an injection for GC separation. As an alternative to membrane separation, the separation can be accomplished by sparging. The subsequent analysis can be done by any detection system, e.g., FID, TOC, MS, FTIR, or GC.
More specifically, the present invention comprises a method for the continuous or discrete monitoring of concentrations at trace levels as low as fractional parts per billion, of TOC/NMOC as well as VOCs, pollutant levels in a fluid stream. The method comprises the steps of collecting at least one sample of the organic compound contaminants using collecting means, from the fluid stream and concentrating said collected samples using concentration means. At predetermined time periods, the concentrated, collected samples are desorbed from the concentration means, using desorption means; and the desorbed, concentrated, collected samples are injected into a detector. The steps are repeated rapidly on a regular and continuing basis in order to provide substantially real time, effectively continuous on-line monitoring. Detectors specifically include FTIR, TOC/NMOC analyzers, GC detectors, e.g., FID, NPD, FPD, TID, TCD, and ECD, mass spectrometers (MS), as well as any sensor in general. In the case of water, the organic compound contaminants are first separated, e.g., by membrane separation or sparging, after which the analysis may be done by any of the above-mentioned detectors or GC-associated detectors.